1. Field of the Invention
The present invention relates to a variable gain amplifying apparatus having a gain control function, and a wireless communication apparatus.
2. Related Art of the Invention
A pre-stage amplifier of a reception device in a radio system represented by cellular phones needs to have low-noise and high-gain characteristics if receiving very weak signals, and the pre-amplifier needs to have low-distortion and low-gain characteristics if receiving large signals.
Particularly, in current mobile communications, since the electric field intensity during reception varies significantly depending on the distance between a base station and a mobile station, the reception device requires a large dynamic range, and consequently the pre-amplifier of the reception device needs to have a gain control function.
FIG. 21 shows the circuit configuration of main parts of a cellular phone terminal in which a conventional variable gain amplifying apparatus having this gain control function is used. The cellular phone terminal of FIG. 21 is used in digital radio communications using modulation systems such as QPSK.
In FIG. 21, the cellular phone terminal is comprised of an antenna 1, a duplexer 2, a transmission circuit portion 3, a variable gain amplifying apparatus 4, a mixer 7, a filter 8, mixers 9, 10, a local oscillator 11 and a phase shifter 12. The variable gain amplifying apparatus 4, the mixer 7, the filter 8, the mixers 9, 10, the local oscillator 11 and the phase shifter 12 constitute a reception circuit portion. The mixer 9, the mixer 10, the local oscillator 11 and the phase shifter 12 constitute an orthogonal demodulator.
The antenna 1 sends a send signal as a send wave, and receives a receive wave as a receive signal. The duplexer 2 guides the send signal outputted from the transmission circuit 3 to the antenna 1, and guides the receive wave received at the antenna 1 to the variable gain amplifying apparatus 4. The variable gain amplifying apparatus 4 is a circuit that amplifies the receive signal as the pre-amplifier of the reception circuit portion. The mixer 7 is a circuit that combines the signal inputted from an oscillator (not shown) with the receive signal amplified by the variable gain amplifying apparatus 4, thereby converting the amplified signal into an intermediate frequency signal. The filter 8 is a circuit that weakens unnecessary frequency components of the signal converted into the intermediate frequency signal. The mixer 10 is a circuit that combines together the signal outputted from the filter 8 and the signal inputted from the local oscillator 11, thereby demodulating a baseband I signal. The mixer 9 is a circuit that combines together the signal outputted from the filter 8 and the signal with its phase shifted by an angle of 90 degrees by the phase shifter 12, thereby demodulating a base band Q signal. The phase shifter 12 is a circuit that shifts the phase of the signal oscillated by the local oscillator 11 by an angle of 90 degrees.
The variable gain amplifying apparatus 4 is comprised of an amplifier 5 and a switching element 6.
The operation of this conventional cellular phone terminal will now be described.
The receive signal received at the antenna 1 is amplified by the variable gain amplifying apparatus 4 via the duplexer 2. The mixer 7 converts the receive signal amplified by the variable gain amplifying apparatus 4 into the intermediate frequency signal, and the filter 8 weakens unnecessary frequency components of the converted intermediate frequency signal.
On the other hand, the signal from the local oscillator 11 is outputted to the mixer 10 and the phase shifter 12. The mixer 10 combines together the output signal from the filter 8 and the signal inputted from the local oscillator 11, thereby demodulating the base band I signal.
The phase shifter 12 outputs to the mixer 9 the signal with its phase shifted by an angle of 90 degrees coming from the local oscillator 11, and the mixer 9 combines together the output signal from the filter 8 and the signal from the phase shifter 12, thereby demodulating the base band Q signal.
The demodulated base band I signal and base band Q signal are inputted to a base band portion (not shown), where they are restored to digital voice data.
If the cellular phone terminal exists within close range of the base station, and so on, the magnitude of the receive signal received at the antenna 1 is large. In this case, a large receive signal is inputted to the variable gain amplifying apparatus 4 as well. In this case, the supply to the variable gain amplifying apparatus 4 of a power voltage for operating the amplifier 5 is stopped, and the switching element 6 is controlled so that it becomes enabled. Therefore, the large receive signal inputted from the duplexer 2 is outputted to the mixer 7 through the switching element 6 without being amplified by the amplifier 5. That is, the variable gain amplifying apparatus 4 operates in a low gain mode. In this way, the variable gain amplifying apparatus 4 operates in a low gain mode when the magnitude of the receive signal is large.
On the other hand, if the cellular phone terminal exists at a great distance from the base station, and so on, the intensity of the receive signal received at the antenna 1 is very low. In this case, a very weak signal is inputted to the variable gain amplifying apparatus 4 as well. In this case, the variable gain amplifying apparatus 4 is supplied with a power voltage for operating the amplifier 5, and the switching element 6 is controlled so that it becomes disabled. Therefore, the very weak receive signal inputted from the duplexer 2 is amplified by the amplifier 5 and then outputted to the mixer 7 without passing through the switching element 6. That is, the variable gain amplifying apparatus 4 operates in a high gain mode. In this way, the variable gain amplifying apparatus 4 operates in a high gain mode when the intensity of the receive signal is very low.
In this way, the variable gain amplifying apparatus 4 operates in a low gain mode when the magnitude of the receive signal is large, and operates in a high gain mode when the intensity of the receive signal is very low, and therefore the variable gain amplifying apparatus 4 can have a wide dynamic range.
However, the inventor has found that the amount of phase shift of the receive signal passed through the variable gain amplifying apparatus 4 when the variable gain amplifying apparatus 4 operates in a low gain mode is different from the amount of phase shift of the receive signal passed through the variable gain amplifying apparatus 4 when the variable gain amplifying apparatus 4 operates in a high gain mode.
Therefore, when the variable gain amplifying apparatus 4 is switched from the low gain mode to the high gain mode, or switched from the high gain mode to the low gain mode, the phase of the signal outputted from the variable gain amplifying apparatus 4 is shifted in a discontinuous manner, thus making it impossible to provide synchronization in the orthogonal demodulator constituted by the mixer 9, the mixer 10, the local oscillator 11 and the phase shifter 12. Therefore, the base band I signal and the base band Q signal cannot be demodulated normally in the orthogonal demodulator until synchronization can be provided.
The above situation will be described more specifically. First, the situation in which the orthogonal demodulator demodulates normally the base band I signal and the base band Q signal will be described.
Assume that the input signal to an orthogonal modulator is expressed by the following equation 1.S(t)=I(t)·cos ωt+Q(t)sin ωt  [Equation 1]
wherein I(t) represents a digital signal taking on a value of +1 or −1, and Q(t) represents a digital signal taking on a value of +1 or −1.
In this case, the local oscillator 11 can be synchronized with the input signal S(t) to the orthogonal modulator, and therefore the local oscillator 11 outputs a signal oscillating at cos ωt to the mixer 10. Therefore, in the mixer 10, the signal outputted from the local oscillator 11 and the input signal to the orthogonal demodulator are combined together, and thus the output signal from the mixer 10 is expressed by the following equation 2.S(t)·cos ωt=I(t)·(1/2)·(cos 2ωt+1)+Q(t)·(1/2)·sin 2ωt  [Equation 2]
This output signal is passed through a low pass filter (not shown in FIG. 21), whereby the baseband I signal expressed by the following equation 3 is demodulated.(1/2)·I(t)  [Equation 3]
On the other hand, the output signal cos ωt from the local oscillator 11 is inputted to a phase shifter 12, and the phase shifter 12 shifts the phase by an angle of 90 degrees, thereby outputting the signal of sin ωt to the mixer 9. In the mixer 9, the signal outputted from the phase shifter 12 and the input signal to the orthogonal demodulator are combined together, and thus the output signal from the mixer 9 is expressed by the following equation 4.S(t)·sin ωt=I(t)·(1/2)·sin 2ωt+Q(t)·(1/2)·(1−cos 2ωt)  [Equation 4]
This output signal is passed through a low pass filter (not shown in FIG. 21), whereby the base band Q signal expressed by the following equation 5 is demodulated.(1/2)·Q(t)  [Equation 5]
In this way, in the case where the input signal S(t) from the orthogonal demodulator can be synchronized with the signal oscillated by the local oscillator 11, the base band I signal and the base band Q signal can be demodulated normally by the orthogonal demodulator.
Examples of values of the demodulated base band I signal and base band Q signal expressed by the equations 3 and 5, respectively, are shown in FIG. 22(A). That is, in FIG. 22(B), the values of I(t) and Q(t) are identical to each other, and change with time from +1 to −1 to +1 . . . FIG. 22(B) shows a plot with the base band I signal on the horizontal axis and the base band Q signal on the vertical axis for given values of I(t) and Q(t). The pair of the base band I signal and base band Q signal shown in FIG. 22 (A) is plotted in the first and third quadrant in FIG. 22(B). When a straight line is drawn along the plotted spots, the line slants at an angle of 45° relative to the horizontal axis.
Here, assume that magnitude of the receive signal received at the antenna 1 is changed, and as a result, the variable gain amplifying apparatus 4 is switched from the low gain mode to the high gain mode, or switched from the high gain mode to the low gain mode. In this case, as described above, the amount of phase shift of the receive signal passed through the variable gain amplifying apparatus 4 when the variable gain amplifying apparatus 4 operates in the low gain mode is different from the amount of phase shift of the receive signal passed through the variable gain amplifying apparatus 4 when the variable gain amplifying apparatus 4 operates in the high gain mode. Therefore, the phase of the input signal S(t) to the orthogonal demodulator is shifted by φ.
Thus, the input signal S(t) to the orthogonal modulator is expressed by the following equation 6.S(t)=I(t)·cos(ωt+φ)+Q(t)sin(ωt+φ)  [Equation 6]
wherein I(t) represents a digital signal taking on a value of +1 or −1, and Q(t) represents a digital signal taking on a value of +1 or −1.
In this case, the local oscillator 11 outputs a signal oscillating at cos ωt to the mixer 10. Therefore, in the mixer 10, the signal outputted from the local oscillator 11 and the input signal to the orthogonal demodulator are combined together, and thus the output signal from the mixer 10 is expressed by the following equation 7.S(t)·cos ωt=I(t)·(1/2)(cos φ+cos 2ωt)+Q(t)·(1/2)·(−sin φ+sin 2ωt)  [Equation 7]
This output signal is passed through a low pass filter (not shown in FIG. 21), whereby the base band I signal expressed by the following equation 8 is demodulated.(1/2)·(I(t)·cos φ−Q(t)·sin φ)  [Equation 8]
On the other hand, the output signal cos ωt from the local oscillator 11 is inputted to a phase shifter 12, and the phase shifter 12 shifts the phase by an angle of 90 degrees, thereby outputting the signal of sin ωt to the mixer 9. In the mixer 9, the signal outputted from the phase shifter 12 and the input signal to the orthogonal demodulator are combined together, and thus the output signal from the mixer 9 is expressed by the following equation 9.S(t)·sin ωt=I(t)·(1/2)·(sin φ+sin 2ωt)+Q(t)·(1/2)·(cos φ−cos 2ωt)  [Equation 9]
This output signal is passed through a low pass filter (not shown in FIG. 21), whereby the base band Q signal expressed by the following equation 10 is demodulated.(1/2)·(I(t)·sin φ+Q(t)·cos φ)  [Equation 10]
In this way, when the mode of the variable gain amplifying apparatus 4 is switched, and as a result, the phase of the input signal S(t) to the orthogonal modulator is shifted, the demodulated base band I signal and base band Q signal are changed to those shown in FIG. 23. Namely, the situation shown in FIG. 22(A) is changed to that shown in FIG. 23(A), and the associated plot shown in FIG. 22(B) is changed to that shown in FIG. 23(B). Specifically, the pair of the baseband I signal and base band Q signal shown in FIG. 23(A) is plotted in the first and third quadrants in FIG. 23(B), but when a straight line is drawn along the plotted spots, the line slants at an angle larger than 45° relative to the horizontal axis.
In this way, provided that the pair of values of the demodulated base band I signal and base band Q signal is expressed by (1, 1) in FIG. 22(A), the pair of values may be shifted to the second quadrant when the mode of the variable gain amplifying apparatus 4 is switched. In some cases, the pair of values may be shifted to the third quadrant.
In this way, when the mode of the variable gain amplifying apparatus 4 is switched, the base band I signal and base band Q signal can be demodulated normally until the local oscillator 11 is synchronized with the input signal to the orthogonal demodulator after the mode of the variable gain amplifying apparatus 4 is switched, and therefore the voice digital signal can be restored normally, which may cause disconnection of received voices of the cellular phone terminal and occurrence of noises.
That is, there is a problem such that the phase of the signal outputted from the variable gain amplifying apparatus is shifted in a discontinuous manner when the gain of the variable gain amplifying apparatus is switched.